1. Field of the Invention
The present invention relates to a method for protecting lignocellulosic products, such as paper, cardboard, and the like, against yellowing, particularly yellowing caused by light and heat. According to such a method, the lignocellulosic product is treated with a brightness stabilizing agent.
The invention also relates to lignin-containing, uncoated papers, which are stabilized against yellowing caused by light and heat, as well as to surface treatment compositions which can be used to protect lignocellulosic products against yellowing.
2. Description of Related Art
As regards related art, reference is made to the following publications:
1. Gratzl, J. S.: Das Papier 39 (1985): 10A, V 14-V23. PA0 2. Fischer, K.: Das Papier 44 (1990): 10A, V 11-V18. PA0 3. Heitner, C.: Chapter 15, p. 192-204, ACS Symposium Series No. 531, ed. C. Heitner, J. C. Scaiano, ACS 1993. PA0 4. Janson, J.: Das Papier 47 (1993): 10A, V47-V52. PA0 5. U.S. Pat. No. 4,474,919
It is well-known in the art that light (UV light in particular), heat, moisture and chemicals can give rise to changes in the brightness of cellulose pulps. Usually, such changes result in reduced reflectivity, particularly in blue light. This phenomenon is known as yellowing and can be caused by various factors depending on which type of pulp is concerned. Heat and damp are the main causes of the yellowing of chemical (lignin-free) pulps, whereas mechanical pulps mostly yellow when they are exposed to light. The yellowing of mechanical pulps also varies depending on the raw material (type of wood), production method (with or without chemical pretreatment), and after-treatment (bleaching with different reagents) used. Thus, for instance, sulphonation and peroxide bleaching greatly increase the susceptibility of pulp to light-induced yellowing.
The yellowing of lignocellulosic pulps and products made from such pulps can be prevented in various ways, for instance by means of impregnation or surface treatment using UV screens, antioxidants, or polymers, or by coating the surface with a coating layer or a layer of non-yellowing chemical pulp.
Many of the additives which have been found to prevent yellowing are expensive or problematic from an environmental point of view; others are only effective when introduced in such large amounts that they may have a negative effect on other properties of the product or are uneconomical.